Sizing is used during a manufacture to impart a degree of hydrophobicity to the paper in order to reduce the tendency for dry paper to absorb aqueous liquid. It is common practice to employ sizing agents, such as alkenyl succinic anhydrides, during paper manufacturing in order to obtain sized paper. Sizing agents are hydrophobic substances and are therefore generally added as aqueous emulsions. Such aqueous emulsions of sizing agents may be employed for engine sizing (often termed internal sizing) or surface sizing.
It is common practice to stabilise aqueous emulsions of sizing agents, such as alkenyl succinic anhydrides, with cationic starch or with anionic emulsifiers. Generally cationic starch functions as a protective colloid to help prevent destabilisation of the dispersed phase sizing agent in the emulsion.
WO 2004/022847 discloses the use of polymers comprising vinylamine units as promoters for the engine sizing of paper. For example, aqueous dispersions which comprise stearyldiketene, cationic starch and polyvinylamine are described therein.
WO 2004/059082 relates to an aqueous sizing composition comprising (a) an emulsion that contains an alkenyl succinic anhydride component suspended in an aqueous polymer solution, and (b) a second component selected from the group consisting of cationic starches, anionic starches, water-soluble polymers, water, and mixtures thereof, wherein the alkenyl succinic anhydride component is sufficiently dilute to enable the sizing composition to impart useful sizing properties to a fibrous substrate.
WO 98/50630 describes a process for the production of paper, board and cardboard involving draining a paper stock containing interfering substances in the presence of a fixing agent which is a reaction product prepared by reacting and amino- or ammonium-containing polymer selected from the group consisting of a polymer containing vinyl amine units, a polyalkylene polyamine, a polyamido amine, a polydiallyl dimethyl ammonium chloride, a polymer containing dialkyl amino alkyl acrylamide units or dialkyl amino methacrylamide units, a polyallylamine, and a dicyandiamide-formaldehyde-condensate, with a reactive sizing agent for paper in a weight ratio of polymer to reactive sizing agent in the range of from 15,000:1 to 1:5.
WO 97/05330 reveals a method in papermaking for improving the sizing efficiency of alkenyl succinic anhydride which comprises adding thereto a synthetic cationic polymer that is reactive with said anhydride. In a preferred embodiment, the cationic polymer is a copolymer of about 50 to about 99 mol % vinyl alcohol and about 50 to about 1 mol % vinyl amine. According to another preferred embodiment, the cationic polymer is a copolymer of about 20 to about 90 mol % acrylamide and about 80 to about 10 mol % vinyl amine.
WO 03/106767 defines an aqueous sizing composition which comprises an aqueous emulsion of ASA in which the ASA incorporates a maximum of 1% by weight of polymeric residues. It is indicated that the ASA distillate maybe emulsified in water together with a starch stabiliser. Examples of suitable stabilisers indicated include cationic and anionic starch, a cationic polyacrylamide or other cationic polymer.
DE 10 2011 101232 is directed to a process of emulsifying sizing agents for paper manufacturing in which the sizing agent is emulsified together with a polymer comprising vinyl amine monomers.
However, it is often difficult to form a stable emulsion of sizing agents, particularly of alkenyl succinic anhydrides. Even with cationic polymers, including conventional polymers of vinyl amine polymers, it is often difficult to consistently and reliably form stable aqueous emulsions of sizing agents, including alkenyl succinic anhydrides.
The inventors of the present invention have discovered that improvements in stability can be achieved when certain aqueous dispersions of water-soluble polymers of N-vinylformamide and/or N-vinylacetamide units are applied before during or after emulsification of the sizing agent into an aqueous liquid.